1. Field of the Invention
The present invention relates to a drive control apparatus for an electrically-driven type extensible/retractable antenna as mounted on, for example, an automobile, etc.
2. Description of the Related Art
An electrically-driven type extensible/retractable antenna mounted on an automobile, etc., is generally so formed that, when an associated motor is rotated in a normal or a reverse direction, a telescopic antenna element is extended or retracted. The antenna element, upon being brought to an extension or retraction motion limit, is not further moved. In this case an excessive load is abruptly applied to the motor. When such a situation is continued, an excessive current continues to flow in the motor winding, causing burning. In order to avoid this, a clutch is usually interposed between the motor side and the antenna side. The clutch is of such a type that a clutch plate on a driving side (a motor side) and a clutch plate on a driven side (an antenna side) are so oppositely arranged that they are slidably moved relative to each other with a given frictional resistance. When the antenna element reaches the extension or the retraction motion limit, the driving-and driven-side clutch plates are subject to a slip motion, temporarily releasing their motor- and antenna-side coupling.
The clutch as set out above produces a louder noise upon the slip motion of its clutch plates and causes a severe friction, resulting in a drop in pressure of the clutch in a relatively short period of time. Therefore, no rotational force is transmitted from the driving-side to the driven-side, thereby a reducing reliable operation over a longer period of time.